The present invention relates to a video and sound recording device with an automatic focusing system.
Ultrasonic wave type automatic focusing devices use sound waves having frequencies above the audible range so that one cannot perceive or hear them, but when they are generated at a predetermined frequency for some time interval, they generate sound waves audible to human ears.
Various cameras with an ultrasonic wave type automatic focusing system have long been put on the market and in the cases of still-cameras the impulse-like audible noise (to be referred to as "ultrasonic frequency noise" in this specification) generated by an ultrasonic transducer especially at the start points of ultrasonic waves present no problem at all in practice. However, in the cases of movie or video cameras which record and reproduce the video and audio signals simultaneously, the ultrasonic frequency noise, which is audible, is picked up by a microphone and recorded. As a result, "chirping" noise is reproduced at a predetermined frequency, thus adversely affecting the reproduced audio signal.
If a noise source; that is, an ultrasonic transducer and a microphone are spaced apart from each other by a sufficient distance, no noise is picked up by the latter or noise can be almost suppressed. However, in practice, they must be spaced apart from each other by at least 20 to 30 cm so that the noise generated by the ultrasonic transducer is inevitably picked up by the microphone.
One of the remedies to this problem is the elimination of noise generated from the ultrasonic transducer, but the present ultrasonic transducers cannot transmit ultrasonic sounds without generating the impulse-like audible noise. Lowering the transmission level will result in lowering the noise level, but the effective measuring range will be shortened accordingly. That is, the camera's performance will be degraded.